by pebe » Fri Jan 17, 2014 3:07 pm
OK. I have now found out from the supplier that their solenoid does not have a return spring; ie. the plunger floats free. There is a hole through the plunger that can be fitted with a pin to retain a washer, and a light spring or disc of lightweight foam rubber could be fitted between it and the coil to keep the plunger extended. The washer would then become the diaphragm whose movement will be seen/felt. The magnets should be removed.
The attached circuit is of the LM555 timer. The ‘on’ time of the solenoid is determined by C1, R1 and the diode. The ‘off’ time is determined by C1, R1 and the potentiometer R2. Therefore R2 will control the overall time of the beat; but the ‘on’ period will stay the same.
With R2 in its mid position, the solenoid will be pulsed on once every 0.77sec, ie. at a rate of 78 beats per min – about the same as a child’s heart rate. The solenoid will be on for about 0.1sec each beat, so the average current through the solenoid should be about 15mA. The supply for the 555 is 4.5Vmin, so if the device is to be battery powered, 4 x AA cells should be used. They can be AA Ni-HM rechargeables. If size is a consideration, then batteries could be reduced to 2 x 14500 (AA size) Li-Ion rechargeable cells at 3.7V each. If you go that route, the Li-Ions should be the ‘protected’ type to prevent over-discharge.
I hope that is what you want. If you want any further details of the circuit, please let me know.
- Attachments
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- 555 pulse solenoid.GIF (3.45 KiB) Viewed 24587 times
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